Steady-state multiplicity in a solid oxide fuel cell: Practical considerations

In this paper we perform a model-based analysis of a solid oxide fuel cell (SOFC) system with an integrated steam reformer and with methane as a fuel. The objective of this study is to analyze the steady-state and transient characteristics of this system. For the analysis, we develop a detailed control-oriented model of the system that captures the heat and mass transfer, chemical kinetics, and electrochemical phenomena. We express the dynamics of the reformer and the fuel cell in state-space form. By applying coordinate transformations to the state-space model, we derive analytical expressions of steady-state conditions and transient behaviors of two critical performance variables, namely, fuel utilization and steam-to-carbon balance. Using these results, we solve a constrained steady-state fuel optimization problem using linear programming. Our analysis is supported by simulations. The results presented in this paper can be applied in predicting steady-state conditions and certain transient behaviors and will be useful in control development for SOFC systems.

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Steady State Energy Behavior for a Coupled Solid Oxide Fuel Cell and Metal Hydride Bed Source

ECS Meeting Abstracts ◽

10.1149/ma2007-01/5/353 ◽

2007 ◽

Keyword(s):

Steady State ◽

Fuel Cell ◽

Solid Oxide Fuel Cell ◽

Metal Hydride ◽

State Energy ◽

Solid Oxide ◽

Oxide Fuel ◽

Energy Behavior

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Progress in Understanding Silica Transport Process and Effects in Solid Oxide Fuel Cell Performance

ASME 2010 8th International Fuel Cell Science, Engineering and Technology Conference: Volume 2 ◽

10.1115/fuelcell2010-33297 ◽

2010 ◽

Author(s):

Paolo R. Zafred ◽

Stephen W. Sofie ◽

Paul S. Gentile

Keyword(s):

Steady State ◽

Fuel Cell ◽

Mechanical Strength ◽

Solid Oxide Fuel Cell ◽

Transport Process ◽

Low Cost ◽

Solid Oxide ◽

Cell Performance ◽

Oxide Fuel ◽

Silica Deposition

One of the enabling technologies required for commercialization of high efficiency solid oxide fuel cell (SOFC) stacks is the development of low cost ceramic refractories capable of withstanding the harsh environment during start-up and steady state operation. Although low density, high purity fibrous alumina materials have been used for more than two decades in manufacturing of SOFC stack components, their low mechanical strength and high cost have precluded their use in the next generation pre-commercial generator modules. A current trend in SOFC stack design is to use high strength, low purity mullite bonded, cast ceramics which can be produced in large volume at a relatively low cost. Sufficient strength is required to provide structural support of the stack and its upper internals in addition to withstanding the severe thermal gradients in both steady state and transient conditions. To reduce costs while achieving suitable mechanical strength, thermal shock, and creep resistance, certain levels of silica and other impurities are present in the refractory ceramic. Silica, however, has been established to poison SOFC anodes thus degrading cell performance and stack life. Therefore, silica transport within the stack has become a dominant issue in SOFC generator design. As a result, an important design requirement for the stack ceramic materials is to develop a fundamental understanding of the silicon species transport process based on refractory composition and gas atmosphere in effort to minimize silicon species volatilization through the porous material. The vaporization behavior of the Al-Si-O system has been investigated in numerous studies and verified experimentally. It is well known that when aluminum silicate components are exposed to a reducing atmosphere, the partial pressure of oxygen is low, therefore this causes formation of volatile SiO(g). This SiO(g) gaseous phase is transported by the fuel stream to the anode/electrolyte interface and electrochemically oxidizes back into SiO2 over the triple phase boundaries (TPB) by the oxygen transported via the fuel cell. This re-deposition process of SiO2, known also as Si poisoning, blocks the reaction of fuel oxidation as it takes over the reactive sites, leading to noticeable degradation in cell performance. In this paper, the status of research on formation of volatile silicon species in aluminosilicate SOFC insulation materials is examined. The formation of volatile SiO(g), SiO(OH)(g), and SiO(OH)2(g) are indicated to facilitate silicon transport in anode fuel streams. Silica deposition is shown to degrade fuel cell anode performance utilizing a novel SOFC silicon poisoning test setup, and silica deposition is only observed on YSZ in the electrochemically active regions of the cell.

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